The present invention is directed to a method, apparatus, and system for monitoring and predicting reactive power output. More particularly, the present invention is directed to a method, apparatus, and system for monitoring present reactive power output in real time and predicting near-term future reactive power output.
Electric power is generated and transmitted through complex regional networks in order to provide electricity to consumers in an efficient and reliable manner. These networks contain hundreds and often thousands of generating sources that supply power to millions of users via one or more transmission systems.
Devices using electricity require a voltage level that is regulated within a certain range. Most of these devices are inductive loads and consume reactive power. Due to the dynamic load size and the need for a regulated transmission system voltage level, power systems must be designed to account for continuously changing demand for reactive power, among many other concerns.
Reactive power generated by power plants is commonly expressed in Volt-Ampere-Reactive (VAR) units. The amount and direction of VAR flow in the transmission system in part determines the voltage profile on the transmission system. Voltage levels may be increased by producing additional VARs at the generating units. Similarly, voltage levels may be decreased by absorbing VARs into the generating units. For regulating transmission voltage levels, it is necessary to understand the phenomena that control and limit the VAR flow between a generator and the transmission system.
Generating units (power plants) include several major parts that may each place limits upon the VAR flow related to the unit. An exemplary power plant is shown in FIG. 1. The power plant includes an alternating current generator 100 that enables the generating unit to either produce or absorb reactive power. The operation of a generating unit requires electricity to support miscellaneous plant equipment, including motors, in the station auxiliary system 110. Often, this electricity is provided by connecting a series of power transformers to the output leads of the generator. For example, as shown in FIG. 1, the station auxiliary system 110 includes station service transformers connected to the output of the generator 100. The internal plant equipment, e.g., the motors, supplied by these transformers must be operated within a certain acceptable voltage range. Maintaining this range may result in a restricted operating voltage range at the generator output terminals. The generating unit must also be connected to a transmission system 130, represented in FIG. 1 as a transmission line, in order to transmit electricity to consumers.
The output terminals of the generator 100 are typically connected to the high-voltage transmission system 130 through a step-up transformer 120. Operating conditions of the generator 100, station auxiliary system 110, and transmission system 130 change continuously and play a significant role in determining the VAR capability of a generating unit.
The generator is the central component of the unit and provides a means by which mechanical energy is converted into electrical energy. The generator includes two primary parts: the rotor and the stator. The rotor includes coils of electrical conductors that form a rotor winding. The stator also contains coils of electrical conductors. The generator output terminals are connected to the stator windings. Mechanical energy turns the rotor inside the stator while a current is passed through the rotor winding, inducing a voltage on the stator winding. By controlling the amount of mechanical energy used in turning the rotor, the real power flow in watts from the stator is adjusted. The rotor winding current, also known as the field current, is used to control the reactive power flow in VARs from the stator. Real or reactive power flows can be increased in the stator by boosting mechanical energy to the rotor or by boosting the field current, respectively. These power flows are often limited by the design parameters of the generator. For example, the generator output, stator and field current limits are due to heating effects associated with increased current.
To counteract these heating effects, hydrogen gas is used in most large generators as a cooling medium. Increasing the gas pressure, and thus the flow, of the cooling medium may increase the stator and field current capability limits.
Generator manufacturers normally express the VAR capability limits of a generator unit in a machine specific capability curve, such as that shown in FIG. 2. FIG. 2 illustrates a typical capability curve as provided by the manufacturer of the generator. For hydrogen cooled machines, a family of capability curves at various hydrogen pressures up to rated pressure may be provided. As shown in FIG. 2, the capability curve shows the leading and lagging MegaVARS (MVARS) for given Megawatts (MWATTS). Plant operators use these types of curves to monitor the output MVARS and MWATTS to ensure that they stay within the capability limits, thereby preventing harm to the plant, while the MVARS output is adjusted to maintain the transmission voltage schedule.
Many devices and software programs have been built to display the capability curve at the rated hydrogen pressure and the present operating point. However, the use of such devices to determine an available VAR reserve is unacceptable, since the true capability may be somewhat less than the rated capability, due to real-time hydrogen pressure, the transmission system voltage level or station service system limitations.
Generating unit operators often operate hydrogen cooled machines at pressures below the rated design conditions to eliminate unnecessary gas leakage. Observing signals from temperature measurement devices within the generator allows the operator to regulate hydrogen pressure to an optimal level. Depending on how far below the rated hydrogen pressure the generating unit is operated, there may be significant sacrifice of reserve VAR capability. This reserve VAR capability may be needed to regulate generator terminal voltage in the event of transmission system changes, such as large load additions, large load trips, transmission line trips or reclosures. Therefore, real-time capability curves at the present hydrogen pressure are needed to estimate the VAR reserve.
Boosting field current to produce VARs generally results in raising the generator terminal voltage magnitude. Decreasing field current to absorb VARs generally results in lowering the generator terminal voltage magnitude. Most generator designs allow a terminal voltage range between 95% and 105% of rated voltage. Therefore, the real time generator terminal voltage is needed to estimate the VAR reserve.
The station auxiliary system voltage must be regulated within a range specified by equipment manufacturer guidelines or established utility procedures. The minimum and maximum acceptable voltages of this system can have a direct impact on the allowable 95% to 105% generator terminal voltage operating range due to the interconnection of these two systems through a unit auxiliary transformer (UAT). Station auxiliary systems commonly restrict this range from its 10% bandwidth to a window of approximately 6-8% (unless load tap changers are used on the transformers). Since the station auxiliary system usually contains a series of transformers to achieve multiple voltage levels, the generator terminal voltage may be limited by equipment at any of these voltage levels. In many cases, equipment specifications on a low voltage bus (480 or 600 V) may restrict the generator operating range. Knowledge of this restriction may allow the plant engineer to design a solution by redistributing loads within the facility.
In order to minimize the limitations on the generator terminal voltage range, optimal transformer taps must be determined through a coordinated study of the entire station auxiliary system. Restriction of the generator voltage range is a function of transformer impedance, voltage ratio, tap selection, and real time loading on the transformer. Therefore, real time loading of the major station service transformers is needed to determine the VAR reserve.
The generator step-up transformer transformation ratio, including tap settings and present transmission system voltage, can have a major impact on the generator""s VAR reserves. Tap settings are established during the initial generating unit start-up and are seldom changed unless significant system changes occur. Transmission system voltages change throughout each day and determine the present VAR flow of a unit given its system design and present operating conditions.
Thus, there is a need for a technique for determining a reactive power output in real time and predicting a future near-term reactive power output.
It is an object of the present invention to provide a technique for determining a reactive power output in real time and predicting a future near-term reactive power output.
This and other objects are met by a method, apparatus, and system for determining an actual real-time reactive power output reserve of at least one generator and predicting a future reactive power output reserve of the generator.
According to a first aspect of the invention, the actual real-time reactive power output reserve of the generator is calculated by determining a real-time reactive power output of the generator at present operating conditions, determining a maximum reactive power output of the generator at the present operating conditions, and subtracting the real-time reactive power output from the maximum reactive power output. The real-time reactive power output may be determined based on a maximum volt-ampere output capability of the generator at the present operating conditions, a real-time transmission voltage level of a transmission system connected to the generator, and a real-time maximum acceptable generator terminal voltage, including any real-time voltage limits associated with station service load buses connected to the generator.
According to another aspect of the invention, the near-term future reactive power output reserve of the generator is calculated by determining an expected maximum reactive power output of the generator at rated operating conditions and subtracting the real-time reactive power output from the expected maximum reactive power output. According to this embodiment, the real time reactive power output may be determined based on a rated maximum volt-ampere output capability of the generator at the rated operating conditions, a scheduled transmission voltage level of a transmission system connected to the generator, and a maximum acceptable generator terminal voltage, including any real-time voltage limits associated with station service load service buses connected to the generator.
According to exemplary embodiments, the determining of the reactive power output reserve may be repeated at regular near real-time intervals. Also, the reactive power output reserve may be determined for a plurality of generators.
Other information and benefits that may be gained from the invention will become more evident from the following description, especially when studying the figures and graphical displays.